In wireless communication networks built on technologies such as Wideband Code Division Multiple Access, W-CDMA, High Speed Packet Access, HSPA and Enhanced Uplink concept, EUL, soft handover, SHO, also referred to as macro diversity, and fast closed-loop power control are essential features for providing uninterrupted and seamless coverage to user equipments, UEs, travelling across cell borders. FIGS. 1a and 1b illustrate a traditional HSPA deployment scenario with a first and a second base station 101, 102 having a similar transmit power level. Ideally, a UE 110 moving from a serving cell of the first base station 101 towards a non-serving cell of the second base station 102 (the movement illustrated with an arrow in FIG. 1a) would enter a SHO region at border A. At border B where the UE experiences approximately the same reception power for signals received from the first base station as for signals received from the second base station, a serving cell change would occur, such that non-serving cell becomes serving cell and vice versa. Further, at border C the UE 110 would leave the SHO region and only have connection to the second base station 102. It is a radio network controller, controlling the first 101 and the second base station 102 that is in control of reconfigurations, which implies rather long delays for e.g. performing such a cell change. During SHO, the UE is essentially power-controlled by the best uplink base station due to the “DOWN-before-UP” principle, i.e. it is enough that one base station indicates a DOWN command for the UE to lower its power. Since the base stations have roughly the same transmit power, the optimal downlink, DL, and uplink, UL, cell borders will coincide at point B. UL cell border is defined as the place where the path loss from the UE to the first base station will be equal the path loss from the UE to the second base station. Hence, in an ideal setting and from a static (long-term fading) point of view, the serving cell would always correspond to the best uplink. However, in practice, due to imperfections, e.g. reconfiguration delays, and fast fading, the UE might be power controlled solely by the non-serving base station during SHO. In such case there might be problems to receive essential control channel information from the UE in the serving base station due to the weaker link between the serving base station and the UE. For example, a Dedicated Physical Control Channel for High Speed Downlink Shared Channel, HS-DPCCH, and scheduling information need to be received in the serving base station. This problem becomes more pronounced in deployments with significant link imbalances, e.g. heterogeneous networks or multiflow.
Deployment of low-power nodes, LPNs, is seen as a powerful tool to meet the ever-increasing demand for mobile broadband services. A LPN may correspond, for example, to a remote radio unit, RRU, pico base station or micro base station, allowing expanding the network capacity in a cost-efficient way. A LPN is defined as having a lower output power than a high power node, HPN. The HPN may be a macro base station in a system where the LPNs are micro or pico base stations. A network consisting of such HPNs and LPNs is referred to as a heterogeneous network. Two examples of use-cases for heterogeneous network deployment that may be envisioned are coverage holes and capacity enhancement for localized traffic hotspots.
Since the LPNs and the HPNs in a heterogeneous network have different transmit powers, the UL and DL cell borders will normally not coincide. Such an example is shown in FIGS. 2a and 2b where the UE 110 when it is in the region between equal path loss border D and equal downlink received power border E has a smaller path loss to the LPN, while the strongest received power is from the HPN. In such a scenario, the UL is better served by the non-serving LPN 101 while the DL is provided by the serving HPN 102. The region between the equal path loss border D and equal downlink received power (e.g. CPICH receive power) border E is referred to as an imbalance region. Further in FIG. 2b there is a SHO region between borders F and G in which the UE is in soft handover, i.e. connected to both the HPN and the LPN. In the region between E and G, the LPN is the serving node. In the region between E and F, the HPN is the serving node. In the imbalance region some fundamental problems may be encountered. For example, a UE in the region between borders E and F, i.e. in the SHO region but also in the imbalance region, would have the HPN 102 as the serving network node, but in general be power controlled towards the LPN 101. Due to the UL-DL imbalance, the UL towards the serving HPN would be very weak, which means that important control information, such as scheduling information or HS-DPCCH, might not be reliably decoded by the serving HPN. Furthermore, a UE in between borders D and F would have the HPN as the serving network node, and also be power controlled towards the HPN (i.e. not in SHO). Due to the UL-DL imbalance, the UE would cause excessive interference in the LPN node.
A current solution to the imbalance problem is to increase the signal strength in UL enough so that the HPN when it is the serving network node can decode the signal also when the UE is in the imbalance region. This may be performed by either increasing the Signal to Interference Ratio, SIR, target in the LPN or by adding noise in the LPN. However, this causes high interference in the LPN.
Consequently, a severe negative impact on DL and UL scheduling is foreseen due to unreliable reception of UL signals when in such imbalance regions. Also, a more unpredictable/uncontrolled interference characteristic in the network is a direct consequence if nothing is done. All in all, a potentially severe network impact and end-user impact can be envisioned. As shown, there is a need for a solution to handle such imbalance problems.